1. Field of the Invention
This invention relates generally to microvalve devices for controlling gas flow on a micro basis and, more particularly, to integrated semiconductor microvalve devices and microflow controllers that are fabricated using semiconductor materials and techniques and by semiconductor-type processing techniques.
2. Discussion of Related Art
Many applications exist in which it is necessary to accurately monitor and/or control very small changes in fluid pressure, especially the change in pressure of a very low pressure fluid stream. Monitoring such pressure changes permits one to measure flow velocity. With knowledge of the stream temperature and dimensions of the stream conduit or channel, one may even be able to use a pressure sensor as a mass flow meter. If a valve is placed in the path of the stream, and is made to be controlled by the pressure sensor, a fluid flow controller results. The use of miniature multi-part assemblies forming such a controller is well known in industry. Microflow meters are especially attractive in the medical and semiconductor process control areas, where gases are relatively clean and flow rates are often low, i.e., between about 0.1 and 10 standard cubic centimeter per minute. In the semiconductor industry, specific applications for these types of flow meters include low pressure semiconductor process control equipment such as ion implanters, chemical vapor deposition (CVD) reactors, sputtering equipment, and the like. Presently, these semiconductor industry applications represent an approximately $50,000,000 per year market for microflow meter applications. However, with increasing use of integrated circuits, and as such equipment becomes increasingly automated, the market is expected to grow significantly.
Currently, conventional microflow meters and microflow controllers do not generally perform satisfactorily below a flow rate of about one standard cubic centimeter per minute (SCCM). These prior art devices are generally constructed of intricate and/or small metal components which require precision assembly. Due to the intricacy of the various parts, and the delicateness with which they must be assembled, the cost of such assemblies is typically high. Further, the result and complex assembly of multiple parts may tend to be unreliable. Unreliability is at least in part due to the corrosiveness of materials used, human error in assembly, and the assembly's number of discrete parts. Still further, these prior art conventional microflow meters are gas velocity flow meters, based either on the measurement of a difference in pressure of a moving gas stream or on the measurement of a difference in temperature of a moving gas stream as the moving gas passes over a heated element. One disadvantage associated with determining difference in gas temperature, as compared to determining a difference in gas pressure, is that the time response of the differential gas temperature devices is slower than that of a differential pressure device. Additionally, it is objectionable and perhaps even unsafe to heat some gases.
Recent developments in thin film deposition and micro machining techniques now permit the reproducible formation of very thin silicon and dielectric integral diaphragms and other configurations, which can comprise extremely small capacitive type pressure sensors. U.S. Pat. No. 4,815,472, issued to Wise el al. and assigned to the assignee of this invention, discloses such a sensor and is herein incorporated by reference. Briefly, the Wise et al. pressure sensor comprises a micro-machined single crystal silicon element, having a very thin integral diaphragm, affixed to a glass plate. The diaphragm and glass surfaces are spaced apart and include complementary electrodes that respectively form opposed plates of a capacitor. Change in pressure flexes the diaphragm, which changes the capacitance between the plates. The change in capacitance is thus a measure of pressure change.
The Wise et al. capacitor pressure sensor can be made using techniques quite similar to and compatible with the techniques typically used in integrated circuit wafer processing. Hence, such a pressure sensor can comprise a silicon substrate that also contains electronic components. The electronic components can be a temperature sensing resistor, diode, transistor and/or other signal conditioning electronics. The resulting integrated device, i.e., the pressure sensor (for a gas in an integral fluid flow channel) and the temperature sensor can provide a microflow meter that can measure pressures from 1 mTORR to over 100 TORR, having an associated flow resolution of 1.times.10.sup.-8 SCCM and a range extending to 1.times.10.sup.-3 SCCM. One such microflow meter, incorporating the capacitive pressure sensor disclosed in the above referenced patent, is disclosed in a paper co-authored by one of the present inventors. The paper is identified by the citation: S. T. Cho, K. Najafi, C. L. Lowman, and K. D. Wise, "An Ultra Sensitive Silicon Pressure-Based Flow Meter", IEDM, pp. 499-502 (December 1989). Such structures are made of materials that are inherently non-reactive with respect to many gases. Also, such structures are substantially monolithic. Hence they are inherently and/or statistically more reliable than the prior art flow control assemblies.
The sensitivity of the microflow meter described in the publications cited above is typically an order of magnitude higher than present commercially available microflow meters. The micromachined integrated microflow meter is significantly more sensitive than present commercially available microflow controllers. There is presently no commercially available way to reliably control the small gas flows which can be measured by micromachined semiconductor type pressure sensors. In order to control microflow of a gas, one must be able to incorporate a microvalve in the gas flow channel. One method proposed in the literature, is to close a small orifice using a cantilevered flap of a semiconductor material. In such proposed designs, an elongated cantilevered flap-like member is disposed adjacent a gas orifice, in position to close the orifice if the flap-like member is moved against the surface defining the orifice. It is proposed to move the member to that surface, and close the orifice, by electrostatic attraction. Thus, when the flap-like member is electro-statically attracted to the surface having the opening, the cantilevered member is (hopefully) moved into complete flush contact with the surface periphery defining the hole. In such a valve structure, the valve is normally open. It is closed by electrostatic action.
This invention recognizes some problems with respect to the prior proposed type of electrostatically actuated microvalve structure. The opening of the microvalve, if closed, and the maintenance of the microvalve in the open position require that the flap-like element have a spring-like quality and/or some inherent stiffness. In order to obtain a quick opening speed, and to ensure that the microvalve stays completely open when not actuated into the closed position, one must make the flap-like element quite stiff. This increases the voltage needed to close it, and reduces the likelihood of obtaining a good seal around the hole, unless the flap-like element has an extended length. On the other hand, if it has extended length, it may not have sufficient rigidity to open quickly and/or stay open. No mechanism has been suggested to solve such problems or to control the oscillations of the cantilevered member when the electrostatic force is released. In other words, the cantilevered member could flap up and down at its natural frequency, thus interfering with gas flow through the associated orifice. Also, the natural spring response of cantilever structures is susceptible to oscillation induced by mechanical movement of the structure. Such problems represent, in our view, significant obstacles to rendering such micromachined, electro-statically driven microvalve structures commercially useful. If a useful and highly sensitive microvalve structure cannot be made, a useful and highly sensitive microflow controller cannot be made.
A micromachined microvalve is described in the publication Ohnstein, et al., "Micromachined Silicon Microvalve", IEEE, pp. 95-98 (April, 1990). The silicon microvalve disclosed in that paper is an electrostatically actuated microvalve which is intended to modulate a gas flow. This microvalve is integrally fabricated on a single silicon wafer using surface and bulk micromachining. The electrostatic actuation of this silicon microvalve is limited to causing a closure plate to electrostatically seal an inlet orifice. In other words, electrostatic action operates to pull the closure plate over the inlet orifice. This proposed structure suffers from the problems referred to above. Therefore, the problems with controlling microflow remain, thus impeding commercial development, including more widespread use, of microflow meters.
Therefore, a primary objective of the present invention is to provide an improved microvalve structure capable of precisely controlling very small fluid flow and overcoming the limitations associated with earlier proposed designs. A principal object of the present invention is to provide a microvalve which utilizes a cantilevered member electrostatically positioned to close a fluid flow orifice as well as to open it.
Yet another object of the present invention is to provide a semiconductor microvalve structure which can be fabricated in multiples, using micromachining techniques, in order to provide a microvalve controller fabricated at a significant cost reduction over its conventional counterpart, and yet have a greater sensitivity and reliability.
Other major objectives include providing several improved integrated silicon semiconductor microvalve structures capable of being mass produced using familiar silicon semiconductor processing techniques to simultaneously construct numerous devices on a silicon wafer.